How to merge two weightpainted meshes with two separate armatures?
After joining the two objects,

To use more than one armature

i) Set up so the body rig is the parent of the mesh and the first armature modifier (armature parent) ... standard setup for one armature.  If you select your body last you will still have this setup on join.

ii) Make the body armature hand pose bones the parents of the glove armatures, arranged to appear as thought it is one rig.  If you have another face rig make the head bone the parent etc.

iii) Add another armature modifier to the modifiers stack for the glove armature.  (One for a face rig etc)

PS. good practice to apply scale to your rigs and meshes so they are all (1,1,1).  Not doing so can cause problems later.